1. Field of the Invention
This invention relates generally to a write clock generator, and more particularly is directed to a write clock generator for use in a digital time base corrector by which time base errors are removed from periodic information signals, such as, color video signals.
2. Description of the Prior Art
Video signals are frequently recorded on magnetic tape and subsequently reproduced for later broadcasting or viewing purposes. During the reproduction of recorded video signals, time base or frequency errors may be introduced as a result of expansion or contraction of the record medium during or after recording, variation in the speed of the tape relative to the magnetic head or heads during recording or reproduction, variation between the tape recording speed and the tape reducing speed, and the like. The presence of such time base errors in the reproduced video signals causes a frequency shift of the latter which can result in many observable undesirable effects, particularly when the reproduced video signals are to be transmitted or broadcast and may be mixed with live broadcast material or other reproduced video signals that do not have such time base errors. The observable undesirable effects resulting from relatively small time base errors are a smeared or jittery picture with erroneous intensity variations and, in the case of color video signals, improper color display. When the time base errors are large, the reproduced picture will fail to lock horizontally or vertically.
In view of the foregoing, it has been proposed, for example, as disclosed in U.S. Pat. No. 3,860,952, or in U.S. Pat. Application Ser. No. 642,197, filed Dec. 18, 1975, and having a common assignee herewith, to provide a time base corrector for processing video signals in which the incoming video signals are converted from analog to digital form and temporarily stored in a memory unit. Time base errors are removed from the video signals by writing the digitized signals in the memory unit at a clocking rate which varies in a manner generally proportional to the time base errors, and by fetching or reading out the stored signals at a standard clocking rate, whereupon the read-out digitized video signals are reconverted to analog form.
In a time base corrector of the above described type, it is known to determine the clocking rate at which the digitized signals are written in the memory unit by means of a write clock generator including a voltage controlled oscillator (VCO) in a phase lock loop by which the oscillator output or write clock pulses, after being suitably divided, are compared, as to phase, with at least the horizontal synchronizing signals separated from the video signals for controlling the VCO on the basis of such comparison. In the case where color video signals are being processed for removing time base errors therefrom, it is further known to control the output frequency of the VCO on the basis of the burst signals, as well as the horizontal synchronizing signals, separated from the incoming color video signals so that more accurate compensation or correction of any time base errors can be achieved by means of the described digital time base corrector. The foregoing write clock generator performs satisfactorily in the time base corrector so long as the time relation between each horizontal synchronizing signal and the onset of the respective burst signal is fixed. However, in practice, a relatively wide variation may exist in the time relation between the horizontal synchronizing signals and respective burst signals of color video signals from various sources, for example, as reproduced by a VTR or as obtained from a color video camera so that, if the input to the time base corrector is constituted by reproduced color video signals from a VTR which are edited or mixed with live broadcast material or color video signals from a camera, there will be a periodic changing or shifting in the time relationship between the horizontal synchronizing signals and the respective burst signals. Further, even if the input to the time base corrector is continuously color video signals from a helical scan-type VTR, the time relation between the horizontal synchronizing signals and the respective burst signals may be changed from time to time as a result of skew jitter appearing in the reproduced color video signals due to longitudinal shrinkage or expansion of the magnetic tape. In any case, changes in the time relation between the horizontal synchronizing signals and the respective burst signals of the color video signals applied to the time base corrector can cause a lockout condition of the phase lock loop included in the write clock generator. When such lockout condition occurs, accurate correction or compensation for any time base errors of the incoming color video signals can no longer be achieved.